1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly to a thin film transistor array substrate and a fabricating method thereof that improves an aperture ratio.
2. Discussion of the Related Art
Generally, a liquid crystal display (LCD) controls light transmittance of liquid crystal material using an electric field to display a picture. The liquid crystal displays are largely classified into a vertical electric field type and a horizontal electric field type depending on a direction that the electric field drives the liquid crystal material.
The vertical electric field applying type LCD drives a liquid crystal material in a twisted nematic (TN) mode where a vertical electric field is formed between a pixel electrode and a common electrode arranged opposite to each other on the upper and lower substrates. The vertical electric field applying type LCD has an advantage of a large aperture ratio while having a drawback of a narrow viewing angle of about 90°.
The horizontal electric field applying type LCD drives a liquid crystal material in an in plane switch (IPS) mode where a horizontal electric field is formed between the pixel electrode and the common electrode arranged in parallel to each other on the lower substrate. The horizontal electric field applying type LCD has an advantage of a wide viewing angle of about 160°.
Hereinafter, the horizontal electric field applying type will be described in detail.
FIG. 1 is a plan view showing a structure of a thin film transistor array substrate in a related art liquid crystal display of horizontal electric applying type LCD, and FIG. 2 is a cross-sectional view of the thin film transistor array substrate taken along cross-sectional line II-II′ of FIG. 1.
Referring to FIG. 1 and FIG. 2, the related art thin film transistor array substrate includes a gate line 2 and a data line 4 disposed on a lower substrate 1 and intersecting each other, a thin film transistor 30 formed at each intersection, a pixel area defined by the intersection structure, a pixel electrode 22 and a common electrode 24 formed at a pixel area to form a horizontal field, and a common line 26 connected to the common electrode 24.
The gate line 2 applies a gate signal to the gate electrode 6 of the thin film transistor 30. The data line 4 applies a pixel signal to the pixel electrode 22 via a drain electrode 10 of the thin film transistor 30. The gate line 2 and the data line 4 are disposed to form an intersection structure so that a pixel area is defined. The common line 26 is disposed in parallel with the gate line 2 having the pixel area therebetween to supply a reference voltage to the common electrode 24 for driving a liquid crystal material.
The thin film transistor 30 allows the pixel signal supplied from the data line 4 to be charged and maintained in the pixel electrode 22 in response to the gate signal applied from the gate line 2. The thin film transistor 30 includes a gate electrode 6 connected to the gate line 2, a source electrode 8 connected to the data line 4, and a drain electrode 10 connected to the pixel electrode 22. Furthermore, the thin film transistor 30 includes an active layer (not shown) overlapping the gate electrode 6 having a gate insulating film 12 therebetween to define a channel portion between the source electrode 8 and the drain electrode 10, and an ohmic contact layer 16 for making an ohmic contact with the source electrode 8 and the drain electrode 10.
The pixel electrode 22 is connected to the drain electrode 10 of the thin film transistor 30 and is provided at the pixel area via a contact hole 20 defined through a protective film 18. Specifically, the pixel electrode 22 includes a first horizontal portion 22a connected to the drain electrode 10 and disposed in parallel with adjacent gate lines 2, a second horizontal portion 22b overlapping the common line 26, and an extended portion 22c provided in parallel to the common electrode 24 disposed between the first and second horizontal portions 22a and 22b. The common electrode 24 connected to the common line 26 is provided at the pixel areas. Specifically, the common electrode 24 is disposed in parallel with the extended portion 22c of the pixel electrode 22 at the pixel area 5.
Accordingly, a horizontal electric field is formed between the pixel electrode 22 to which a pixel signal is supplied via the thin film transistor 30 and the common electrode 24 to which a reference voltage is supplied via the common line 26. Specifically, the horizontal electric field is formed between the extended portion 22c of the pixel electrode 22 and the common electrode 24. Liquid crystal material arranged in the horizontal direction between the thin film transistor array substrate and the color filter array substrate rotate due to a dielectric anisotropy of the horizontal electric field. Transmittance of light through the pixel area 5 is varied depending upon a rotation extent of the liquid crystal material, thereby implementing a gray level scale.
The common electrode 24 adjacent to the data line 4 has a width of more than about 10 μm. Accordingly, an electric field formed between the common electrode 24 and the pixel electrode 22 is maintained and not influenced by a data signal of the data line 4. In other words, an alignment of the liquid crystal material disposed in an area between the common electrode 4 adjacent to the data line 4 and the pixel electrode 22 is influenced by the data signal, which changes the light transmittance within the instant area, thereby generating a vertical cross talk. To prevent this, a width of the common electrode 24 adjacent to the data line 4 is formed having an increased width to shield the data signal from the vertical cross talk. However, such an increased width of the common electrode 24 raises a problem in the aperture ratio reduction.
Furthermore, the common electrode 24 and the pixel electrode 22 are disposed in parallel with each other in a direction parallel to the data line 4. The common line 26 supplying a reference voltage to the common electrode 24 and the gate lines are formed simultaneously. The common line is disposed in a direction parallel to the gate line 2. The common line 26 occupies a portion of the pixel area 5, thereby reducing the total pixel area and causing deterioration of the aperture ratio.
Moreover, the liquid crystal material between the pixel electrode 22 and the common electrode 24 is rotated in a predetermined direction by the applied electric field. However, the liquid crystal material disposed between driving electrodes (i.e., the common electrode 24 adjacent to the gate line 2 and the pixel electrode 22) is rotated in a different direction from the pixel area 5 due to the difference in electric field between the gate line 2 and the driving electrode. As shown in FIG. 3, when the pixel area displays black, light leaks in an area between the gate line 2 and the driving electrode. Accordingly, no electric field is applied between the common electrode 24 and the pixel electrode 2. To prevent this, the driving electrode and the gate line 2 are spaced by a distance “d” of about 25 to 28 μm. Since this distance “d” is formed at each side of the pixel area, there raised a problem in that deterioration of an aperture ratio by the distance “d”, may be as much as 56 μm. In addition, the pixel electrode 22 and the common line 26 provided in the pixel area to shut off the gate signal are occupying the separate portions of the pixel areas, thereby deteriorating the aperture ratio.